Hiss/Whoosh Sound ST-120

I have a me-built ST-120. It worked first time powered up. After about 18 months the WZ68 rectifier failed, blowing the fuse. Not having a spare, I soldered (just) diodes into the base of the WZ68 and it has worked fine since.

Tube bias has always been stable, but recently I've noticed all 4 tubes sneak up by about 10% to around 55 on my meter.

Now when I turn it on (with the preamp powered down) the left channel has a hiss that sounds exactly like a tuner between stations. Powering up the preamp and turning the volume up makes no difference. Listening to music (1969 vertical Cornwalls) it sounds fine since the hiss is overwhelmed by almost anything.

Ideas?

Thanks.

IDont know if this relates, but-
My ST120 had a whooshing sound on one channel. The volume was also significantly lower on that side. At the time Bob said it could be the LM336 IC on the VTA board. I change it and this solved my issue. Have a look through the threads if you haven’t.

I thought I'd swap the power tubes from one side to the other, rebias, and see if the wooshing sound moved over to the other side. Instead, it is gone. So maybe it was a dirty socket?

Yes, Swap all tubes between amp/channels to eliminate a bad tube.

Also try tapping each tube when listening for the sound.

As always CLEAN all tube sockets!

Buck

krcrisp wrote:I have a me-built ST-120.  It worked first time powered up.  After about 18 months the WZ68 rectifier failed, blowing the fuse.  Not having a spare, I soldered (just) diodes into the base of the WZ68 and it has worked fine since.

Tube bias has always been stable, but recently I've noticed all 4 tubes sneak up by about 10% to around 55 on my meter.

Now when I turn it on (with the preamp powered down) the left channel has a hiss that sounds exactly like a tuner between stations.  Powering up the preamp and turning the volume up makes no difference.  Listening to music (1969 vertical Cornwalls) it sounds fine since the hiss is overwhelmed by almost anything.

Ideas?

Thanks.

If you're using just diodes in a circuit designed for a tube rectifier, your plate voltage is going to run sky high, shortening tube life, probably by a lot, regardless of whether it was the source of your hiss. The original WZ68 "Copper Cap" includes at least a thermistor for soft start, and I think a dropping resistor as well. Another thing that's going to be stressed is the filter capacitors, already at a slim margin when running at the current US standard line voltage of 120 instead of 115 or 117. Believe me, I know all this from experience.

Personally, I dislike the idea of letting the plate voltage sag, using a tube rectifier or simulated one, for a "warmer" sound. At the very least, you should use a Variac to take down the plate voltage to normal levels. Also, if you're not using a time delay board, you should install one or you'll have "cathode stripping", another tube life shortener.

I thought the VT-120 was a modern design made for 120V line and SS rectifiers?

Buck

Buck_R0gers wrote:I thought the VT-120 was a modern design made for 120V line and SS rectifiers?

Buck

I don't have first hand experience with it, but not if it had a WZ68. My M125's definitely weren't.

After the WZ68 failed and blew the fuse, I looked around on this forum and saw the post from the moderator along the lines of "if you want to try ss rectification, do this...." and I did it as suggested, then adjusted the bias. I assumed Bob wouldn't suggest it if it was a bad idea. I have the time delay installed; it takes 19 seconds to click on.

My line voltage is 119V as of today. I do have a variac. The copper cap that Webber recommends for hifi is just diodes. I do have a new WZ68 I could put back in. The 18 month life on the prior one did not impress me.

OK, and please excuse my ignorance, but how do I test the plate voltage and what should it be? I assume a pin of a power tube to chassis but I don't know which one.

As I'm using a pair of Cornwalls I am really not using the power of the amp much.

Thanks.

OK, I am measuring pin 3, I think. What is OK vs alarmingly high voltage?

"The copper cap that Webber recommends for hifi is just diodes."

I got the WS-1t. The "t" is an added thermistor.  I have the auto-bias board which has a 50 second delay.

As for the transformer voltage, I think Bob upped the spec in the last year or two but you'll have to do a search as it was mentioned somewhere on the forum. My ST-120 is not 120v. I use a variac set to 117v. My wall current fluctuates around 122.9 VAC.  

And yes, clean your sockets and pins!

krcrisp wrote:After the WZ68 failed and blew the fuse, I looked around on this forum and saw the post from the moderator along the lines of "if you want to try ss rectification, do this...." and I did it as suggested, then adjusted the bias.  I assumed Bob wouldn't suggest it if it was a bad idea.  I have the time delay installed; it takes 19 seconds to click on.

My line voltage is 119V as of today.  I do have a variac.  The copper cap that Webber recommends for hifi is just diodes.  I do have a new WZ68 I could put back in.  The 18 month life on the prior one did not impress me.

OK, and please excuse my ignorance, but how do I test the plate voltage and what should it be?  I assume a pin of a power tube to chassis but I don't know which one.

As I'm using a pair of Cornwalls I am really not using the power of the amp much.

Thanks.

If Bob recommends it I defer to his wisdom. My assumption was that the ST120 is an old Dynaco design (not to be confused with the Dynaco Stereo 120, which was a solid state), based on tube rectifiers.

You built it, so I assume you are comfortable measuring potentially lethal voltages, right? But the manual that came with it should specify normal voltages.

One thing that should be easy to do is measure the voltages to ground on all the terminals of the filter caps, and compare that to the working voltage marked on the caps. Do you have antique replica multi-section can capacitors? The "Authenticaps" in my M125's did not at all appreciate being run without the power tubes plugged in, to test the power supplies (contrary to Bob's instructions) and I ended up frying one.

Weber recommends the W68 for guitar amps, where the amp is part of the instrument and the rectifier voltage drop contributes to the sound, and the one without dropping resistors for hi fi, because hi fi users presumably prefer an uncolored sound (and I am definitely in that camp). But in making the substitution you have to make sure your amp can take the inevitably higher voltage that results.

schematic and voltage checks are in the ST120 manual you would have gotten with purchase.
Check the voltages on the quad cap as below, with a SS rectifier voltages will be about 25v higher than listed,
and closer to 560v during the first minute before the output tubes start drawing working current.
Weber copper caps are well known to overheat and fail, you're better off just adding your own diodes and a thermistor, such as CL90 or CL120,
plus using the TDR (time delay relay).
Note # 2 – Line voltages vary slightly throughout the country. Differences in some GZ34 rectifiers can cause variations also.
If a 5U4 rectifier is used in place of the GZ34 then some voltages (marked with an asterisk) will be near the low end of the range.
If a solid state rectifier is used in place of the GZ34 then some voltages (marked with an asterisk) will be at the high end of the voltage range.
GZ34, solid state rectifier or 5U4 rectifier
Pin 4 to ground – 400 - 425 volts AC
Pin 6 to ground – 400 - 425 volts AC
Pin 2 to ground – 490 – 530 volts DC *
Pin 8 to ground – 490 – 530 volts DC *

The measurements of pins on the rectifier are as follows:

Pin 4 402V AC
Pin 6 402V AC
Pin 2 529V DC
Pin 8 527V DC

Is this a problem?

Thanks.

voltages look good